Preserve updated values when exporting environment settings

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I run Live Tennis API.

Loading value = ${TOKEN|old} and then value = replacement updates Get, but WriteTo exports the old expression. Reloading that output restores old.

Discard cached expressions when values or sections change. Cache entries use both the section name and key, so names such as a_b.c and a.b_c cannot collide. Export uses a copy of the data to preserve untouched expressions without changing current values.

The tests cover each update method and deletion. They also check case handling, rejected writes and colliding names. The original code fails the replacement regressions.

Validation: the full suite and race checks pass. Build and vet pass with Go 1.19.13, which also passes the suite. Staticcheck reports the same 131 existing diagnostics as the baseline. Golint reports its existing EncodeOptions comment warning.